Recently, a copper clad laminate comprising an aromatic polyamide fiber having an excellent heat resistance and dimension stability as the base material has been developed and has been used as a substrate for various circuit boards for which a high heat resistance and dimensional stability are required. This copper clad laminate is prepared by laminating a copper foil and at least one prepreg formed by impregnating a woven fabric, paper-like sheet or nonwoven fabric composed of aromatic polyamide fibers, with a varnish composition comprising an epoxy resin or the like.
For example, Japanese Unexamined Patent Publication No. 61-100446 discloses a flexible printed circuit board characterized in that an electrically insulating material formed by impregnating a nonwoven fabric composed mainly of aromatic polyamide fibers with a resin composition comprising an epoxy resin and a rubbery resin is used as a base material and/or cover layer. Furthermore, Japanese Unexamined Patent Publication No. 62-283695 discloses a flexible circuit board characterized by comprising a flexible base plate formed by impregnating an aramid fiber fabric base material having a thickness of 0.025 to 0.5 mm with an adduct of an aromatic amine curing agent to an epoxy resin and a metal foil circuit layer formed on one or two surfaces of the flexible base plate.
Often, however, it was found that when silver electrodes are formed on a substrate as mentioned above and an electric field is applied between these silver electrodes, after a lapse of a predetermined time, a migration of silver into the substrate occurs, especially under high-temperature and high-humidity conditions, resulting in a reduction of the insulation resistance of the substrate. It is considered that this phenomenon of the migration of silver is due to the synergistic action of the aromatic polyamide fiber, which is a material having a high hygroscopic property and containing large quantities of sodium and chloride, and the epoxy resin which is a matrix containing large quantities of sodium and chlorine. A reduction of the insulation resistance between electrodes is a fatal defect in a high-density wiring board such as a COB, and accordingly, the development of a prepreg for a copper clad laminate, in which the reduction of the insulation resistance does not occur, is needed.
Furthermore, a higher heat resistance is required for the substrate, to improve the dimension stability in a wiring board product, and since the wiring circuit width is reduced, the heat resistance should be improved without a reduction of the peel strength. Therefore, the development of a prepreg for a copper clad laminate, which has an excellent property of preventing the migration of silver even in an electric field under high-temperature and high-humidity conditions, and a high heat resistance and peel strength, is urgently required.